Vitamin D and Its Analogues Decrease Amyloid-ß (Aß) Formation and Increase Aß-Degradation.

Alzheimer's disease (AD) is characterized by extracellular plaques in the brain, mainly consisting of amyloid-ß (Aß), as derived from sequential cleavage of the amyloid precursor protein. Epidemiological studies suggest a tight link between hypovitaminosis of the secosteroid vitamin D and AD. Besides decreased vitamin D level in AD patients, an effect of vitamin D on Aß-homeostasis is discussed. However, the exact underlying mechanisms remain to be elucidated and nothing is known about the potential effect of vitamin D analogues. Here we systematically investigate the effect of vitamin D and therapeutically used analogues (maxacalcitol, calcipotriol, alfacalcidol, paricalcitol, doxercalciferol) on AD-relevant mechanisms. D2 and D3 analogues decreased Aß-production and increased Aß-degradation in neuroblastoma cells or vitamin D deficient mouse brains. Effects were mediated by affecting the Aß-producing enzymes BACE1 and γ-secretase. A reduced secretase activity was accompanied by a decreased BACE1 protein level and nicastrin expression, an essential component of the γ-secretase. Vitamin D and analogues decreased ß-secretase activity, not only in mouse brains with mild vitamin D hypovitaminosis, but also in non-deficient mouse brains. Our results further strengthen the link between AD and vitamin D, suggesting that supplementation of vitamin D or vitamin D analogues might have beneficial effects in AD prevention.

Eicosapentaenoic acid and docosahexaenoic acid increase the degradation of amyloid-ß by affecting insulin-degrading enzyme.

Omega-3 polyunsaturated fatty acids (PUFAs) have been proposed to be highly beneficial in Alzheimer's disease (AD). AD pathology is closely linked to an overproduction and accumulation of amyloid-ß (Aß) peptides as extracellular senile plaques in the brain. Total Aß levels are not only dependent on its production by proteolytic processing of the amyloid precursor protein (APP), but also on Aß-clearance mechanisms, including Aß-degrading enzymes. Here we show that the omega-3 PUFAs eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) increase Aß-degradation by affecting insulin-degrading enzyme (IDE), the major Aß-degrading enzyme secreted into the extracellular space of neuronal and microglial cells. The identification of the molecular mechanisms revealed that EPA directly increases IDE enzyme activity and elevates gene expression of IDE. DHA also directly stimulates IDE enzyme activity and affects IDE sorting by increasing exosome release of IDE, resulting in enhanced Aß-degradation in the extracellular milieu. Apart from the known positive effect of DHA in reducing Aß production, EPA and DHA might ameliorate AD pathology by increasing Aß turnover.